If a new toilet stool employed to replace an old one differs in specifications from the old one and has a discharge port differing from that of the old one in its position as viewed along the length of the toilet stool, its installation often presents a number of problems including its interference with a wall of a toilet and its installation in an extremely forwardly protruding position, since a drain in a floor of a toilet is fixed in position. In order to avoid those problems, there is available an adjustable drain socket composed of three separate parts to absorb a difference from one toilet stool to another in distance between its discharge port and the drain in the floor, as disclosed in, for example, JP-A-8-49281.
While the drain socket disclosed in the above publication enables a new toilet stool to be installed without interfering with the wall, it has been useful to absorb only a limited difference from one toilet stool to another in distance between its discharge port and the drain in the floor. Accordingly, there have been desired a drain socket which can absorb a larger such difference, and also a drain socket which can achieve an improved siphoning performance to flush the toilet stool effectively with a smaller amount of water.